This invention relates to the measurement of magnetic signals produced by the brain, and, more particularly, to measurement of visually induced responses of the brain.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain. The biomagnetometer is necessarily a very sensitive measurement instrument, because the magnetic fields produced by the brain are small. The strength of the magnetic field produced by the brain is typically about 0.000000001 Gauss, at a distance of 1-2 centimeters from the head. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or about five hundred million times larger than the strength of the magnetic field of the brain, as measured externally to the head.
The biomagnetometer includes a magnetic pickup coil connected to a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting QUantum Interference Device or SQUID, which, in combination with a superconducting pickup coil, is sufficiently sensitive to detect magnetic signals produced by the brain. The detector, pickup coil, and their associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body. The dewar is operated with its interior at liquid helium temperature (about 4.2K), to maintain the SQUID detector, the pickup coil, and the electrical connection between them in the superconducting state because of the small electrical currents involved, and to reduce the electrical noise that might otherwise influence the SQUID detector.
Special electronics is provided to filter out external effects such as the earth's magnetic field and other electrical sources. The subject and the detector can be placed into a magnetically quiet enclosure that shields the subject and the detector from the external magnetic fields. With these special provisions, medical researchers and doctors can now make accurate, reliable measurements of the magnetic fields produced by the brain in response to external stimuli.
One of the difficult problems in performing measurements of responses to visual stimuli is delivery of those stimuli to the eyes of the subject in a manner that does not, in itself, cause a signal in the biomagnetometer that interferes with its operation, obscures the responses of the subject, or is subject to misinterpretation as a response of the subject. Most electrical equipment produces magnetic fields, in many cases much larger than that of the earth's field or any brain signal that might result. A device to deliver visual stimuli to the subject under study is most conventionally placed in close proximity to the subject. Where a magnetically shielded enclosure is used, it is desirable to place the device inside the enclosure.
There is a need for a visual stimulation device that is magnetically quiet in the sense that it does not produce signals in the biomagnetometer that interfere with its operation or with the interpretation of the subject's response. The device should also be sufficiently versatile to deliver a variety of visual stimuli in an otherwise uniform manner, so that the effects of such different types of stimuli can be evaluated. The present invention fulfills these needs, and further provides related advantages.